Extensions de protocoles contre extensions de types

protocol Greeter {
    func greet() -> String
}
struct Person: Greeter {
    var name: String
    func greet() -> String { "Hi, I'm \\(name)" }  // concrete requirement impl
}
print(Person(name: "Ada").greet())

protocol Describable {
    func describe() -> String
}
extension Describable {
    // default for ALL conformers until they override
    func describe() -> String { "No description" }
}

struct Car: Describable { }             // uses default
struct Book: Describable {
    func describe() -> String { "A book" } // custom override
}
print(Car().describe(), Book().describe())

struct Point { var x: Int; var y: Int }

extension Point {
    // add helper just to Point
    var lengthSquared: Int { x*x + y*y }
    func moved(dx: Int, dy: Int) -> Point { Point(x: x+dx, y: y+dy) }
}
let p = Point(x: 2, y: 3)
print(p.lengthSquared, p.moved(dx: 1, dy: -1))

protocol Summable { }
extension Int: Summable { }
extension Array where Element: Summable {
    // only for arrays whose elements conform to Summable
    func joinedCount() -> Int { self.count }
}
print([1,2,3].joinedCount())   // works (Int conforms)
 // print(["a","b"].joinedCount()) // not available

protocol Speaker { func speak() -> String }
extension Speaker { func speak() -> String { "default" } }

struct Cat: Speaker { }
struct Dog: Speaker {
    func speak() -> String { "woof" } // requirement impl overrides default
}

let s1: Speaker = Cat()
let s2: Speaker = Dog()
print(s1.speak()) // "default" (uses default impl)
print(s2.speak()) // "woof" (Dog provides requirement impl)

let c = Cat()
print(c.speak())  // "default" via extension on protocol